The present invention relates to halftoning and, in particular, to the halftoning of images intended for display on a video display having limited display characteristics.
Conventional liquid crystal displays (LCD""s) for the display of video images normally display an array of dots at a fixed refresh rate. For example, one common form of display is a 640xc3x97480 display format. Video distribution standards, such as NTSC, assume a certain number of images will be presented every second. For example, a common rate is 60 fields per second which corresponds to a pixel update rate of 16.6 ms per pixel. Unfortunately, the response speed of currently available liquid crystals is highly variable and highly non-linear. For example, for a standard liquid crystal display, FIG. 1 illustrates observations made of the time for driving pixels of a continuous tone display from one intensity state to a second intensity state. It can be seen from examination of this data that, for a number of pixels the response time of the display is in excess of 16.6 ms. As a result, significant artefacts can be produced when a large amount of motion is present in a video signal when utilised to drive an LCD type display. The artefacts are often distractingly prevalent to the eye, especially where non-natural imagery is being displayed for example, moving objects having sharp borders, etc.
It is an object of the present invention to provide an alternative form of driving a display having limited response time characteristics so as to lead to improved display of images.
In accordance with a first aspect of the present invention, there is provided a method of halftoning input image data intended for reproduction on a display having a limited pixel response time comprising, for each pixel of the display, the steps of:
in a first halftone cycle, (first) halftoning an input value to display an extreme representable value; and
in a second halftone cycle, (second) halftoning the input value to display an intermediate value such that the average of the extreme representable value and the intermediate value is substantially equal to the input value.
Preferably, each of the pixels of the display includes at least two sub-pixels which are driven with out-of-phase halftone cycles and adjacent pixels are driven in halftone cycles which are again out-of-phase with respect to one another such that, in each halftone cycle, there is formed a checkerboard pattern of extreme values of portions of the display.
Further, preferably the extreme values comprise a fully on pixel value and a fully off pixel value.
The method further preferably includes the step of detecting when the input value has changed around a midpoint value and altering the intermediate value to take into account the change so as to substantially maintain the average and further, overdriving the pixels during a fixed period for display of the pixels so as to produce a predetermined output intensity value of the pixels.
Other aspects of the invention including apparatus, system and computer readable medium are also disclosed.